The recent proliferation of Wi-Fi access points in wireless local area networks (WLANs) has made it possible for positioning systems to use these access points for position determination, especially in areas where there is a large concentration of active Wi-Fi access points (e.g., urban cores, shopping centers, office buildings, and so on). For example, a client device or station (STA) such as a cell phone or tablet computer can use the round trip time (RTT) of signals transmitted to and from the access points (APs) to calculate the distances between the STA and the APs. Once the distances between the STA and three APs are calculated, the location of the STA can be estimated using trilateration techniques. Similarly, the distance between a pair of STAs may also be calculated using the RTT of signals transmitted between the STAs, and thereafter trilateration techniques may be used to determine and/or verify the locations of the STAs.
For example, referring to FIG. 1, the distance (d) between an access point (AP) and a mobile station (STA) may be calculated as d=c*tp/2=c*(RTT-TAT)/2, where c is the speed of light, tp is the summation of the actual signal propagation times of a request (REQ) frame and an acknowledgement (ACK) frame exchanged between the AP and the STA, and TAT is the turn-around time (or processing delay) corresponding to the time delay between the STA receiving the REQ frame from the AP and beginning its transmission of the ACK frame to back the AP. The value of RTT may be measured by the AP as the difference between (1) the time of departure (TOD) of the REQ frame from the AP and (2) the time of arrival (TOA) of the ACK frame at the AP. The value of TAT is typically estimated.
Multiple input-multiple output (MIMO) technology may increase data throughput by allowing wireless devices (e.g., APs and STAs) to transmit data using multiple transmit chains and antennas. For MIMO devices, cyclic shift diversity (CSD) may be introduced between the transmit chains to prevent beam-forming by the multiple antennas. However, when performing ranging operations between wireless devices, phase delays associated with the CSD introduced between the transmit chains in the STA may result in errors associated with the measured TOA values of received signals. For example, referring again to FIG. 1, if the STA uses multiple transmit chains to send the ACK frame to the AP, then the CSD between the multiple transmit chains in the STA may result in different TOA values measured by the AP. A number of post processing operations may be used to compensate for the different TOA values resulting from CSD introduced by the STA when transmitting the ACK frame using multiple transmit chains. In addition to consuming power and processing resources, these post processing operations may not accurately compensate for CSD.
Thus, there is a need to increase the accuracy of RTT ranging operations performed between two wireless devices.